Yep, Sorcerer is right.
In short: if you force-annex a nation you will get at least 5 BB points (correct me if 6 is the minimum and don't start with religion and ROTW

)
By doing nothing your BB decreases by 1 every 4 years (depends on version iirc).
So waiting for decades, that's right, that's what you have to do to lower your BB besides loosing a province in a peace treaty (BB reduced by 1), but that's something I'm sure you don't want to do.
What can you do to prevent this?
1. Diploannexing. You will still get some BB points, but less.
2. Let others do the annexing and then take the provinces from them without annexing them.
3. Let other nations be the big badboy. Then you can do whatever you want. BB wars are fought against the nation with the highest BB. Just stay a bit behind them (very often the big bad boy is Turkey, Spain, or France).
You can check the badbody value for each nation by opening your save game with a text editor. Search for "badboy".